This study is a new proposal concerned with a behavioral intervention, that of taking an afternoon nap, which might lessen some of the sleep and alertness disorders experienced by many older men and women. The major aim of this experiment is to determine whether for older people (70y+), a daily 90-minute early afternoon "siesta nap" regimen, applied for 2 weeks in the home followed by 3 days in the laboratory, will help promote evening alertness and prevent Unwanted Early Evening Sleepiness (UEES) which may be manifest as evening napping and/or Advanced Sleep Phase Syndrome (ASPS) symptoms. A within-subject design will be employed with all subjects doing both conditions, half of the subjects doing the 'nap' condition first, half the 'no-nap' condition first. Eighteen days will intervene between the two conditions. The applicants will study both healthy controls and sleep disorders patients who complain of UEES symptoms, expecting the intervention to be most beneficial for the patient group. Home-based measures will include subjective ratings of alertness, sleep (and nap) diaries, the Social Rhythm Metric (SRM), and Actillume measures of wrist activity and light level. Laboratory measures will additionally include rectal temperature, polysomnographic measures of naps and sleeps, and single MSLT and waking EEG evening measures of sleepiness. For both patient (10m, 10f) and control (10m, 10f) groups, the applicants will test the hypothesis that the siesta nap regimen, imposed for 14 days at home and for three days in the laboratory, will have the effect of: 1) delaying the self selected bedtime and/or eliminating evening naps, 2) improving evening performance at manual dexterity, serial response, response inhibition and monotonous visual vigilance tasks, 3) increasing evening subjective alertness (self- rated) and decreasing objective evening sleepiness (MSLT trial, waking EEG analysis), 4) changing the pattern of light exposure and 5) increasing the level of lifestyle regularity. The intervention is derived from Borbely's original "Process S/Process C" model of the human circadian system, and laboratory sleep and circadian measures will be used additionally to investigate the mechanisms of the model, and to evaluate any consequences of the siesta naps on circadian rhythm timing and nocturnal sleep. Results from the experiment will reveal the potential benefits (and costs) to older people of a lifestyle that incorporates an early afternoon nap, and determine how these vary between healthy seniors and those complaining of symptoms related to unwanted early evening sleepiness.